The present invention relates generally to the field of alumina ceramics and, more particularly, to a method and an apparatus for making transparent alumina ceramics using microwave sintering.
Transparency is a valuable optical property in certain optical materials. In polycrystalline materials, a number of factors influence the degree of transparency of the material including grain size, density, crystal structure, porosity, and the grain boundary phase. Glasses are optically isotropic, monolithic (i.e. have no grain boundaries), and therefore possess excellent transparency. So can cubic ceramics. All non-cubic ceramics are anisotropic and polycrystalline. The grain boundaries in the ceramic strongly scatter light. However, if the grain size is smaller than the wavelength of the light (0.4-0.7 xcexcm), then light can transmit through the ceramic. If the grain size is larger than the wavelength of light, by minimizing grain boundaries and impurity content, the material can be made transparent or translucent.
To achieve transparency in a ceramic, one must control grain growth, eliminate porosity, and achieve a fully dense material. Conventional methods of fabricating fully dense and reasonably transparent ceramics involve high temperatures, lengthy sintering times, and various complex processing steps, which not only make the processing of transparent ceramics uneconomical but often the desired properties are not achieved.
Transparent alumina (Al2O3) ceramics can be prepared either in single crystal (have generally termed, sapphire) or polycrystalline forms. Sapphire is used in many industrial and military applications, such as optical windows for lasers, spectrometers, armor parts, and IR-domes for infrared missile guidance systems. Also, synthetic sapphire gemstones have become a popular jewel material. Other common alumina ceramics include, for example, MgAl2O4 (spinel) and (Na,Ca) Al12O19 (xcex2-alumina). Polycrystalline transparent alumina for optical applications was first made in the early 1960s as described in U.S. Pat. No. 3,026,210, issued to Coble. In Coble, polycrystalline alumina bodies having the desired optical properties were made by preparing a mixture of high purity finely divided alumina powder with {fraction (1/16)} to about xc2xd weight percent of finely divided magnesia (MgO). The method comprised compacting the mixture of finely divided alumina and magnesia, and firing the compact for predetermined periods of time at a temperature not lower than about 1700xc2x0 C. in a vacuum or a hydrogen environment. The resultant polycrystalline transparent alumina has become a key element in high-pressure sodium vapor lamps and other optical instruments manufactured throughout the world. The cost to manufacture polycrystalline transparent alumina is much lower than that of sapphire and it is easier to produce in large size products.
As described in Coble, and in techniques which have become well known in the art, polycrystalline transparent alumina is made via powder processing using high purity and fine particle sized alumina powder with the addition of a small amount of MgO, and sintering to pore-free state. Sintering is essential in obtaining high transparency material. In the conventional sintering process, extremely high sintering temperatures (up to 1900xc2x0 C.) and long soaking times (several hours) under high vacuum or pure hydrogen atmosphere are applied in the fabrication of transparent alumina products to achieve the highest density and minimum porosity.
Microwave sintering is a new technique for ceramic materials processing which differs fundamentally from current conventional processes just described. In microwave processing, samples positioned in a microwave field absorb microwave energy and convert it into heat directly providing volumetric heating. As a result, a microwave process provides several advantages, such as more rapid and uniform heating, shorter processing time, fine microstructure, enhanced energy efficiency, and improved materials properties and product performance. Enhanced densification behaviors are also provided when microwave processes are used due to a reduction in the activation energy for sintering, which leads to a lower sintering temperature and shorter sintering time compared to conventional sintering processes.
Microwave sintering of alumina material is known. The early work on microwave sintering of alumina ceramics was performed in 1975 and reported by W. H. Sutton. In that work, over 1360 kg of production shapes of alumina castables were successfully fired using microwave energy. J. Katz et al. reported the successful sintering of relatively large samples (about 1 kg) of high purity, undoped Al2O3 to about 93% theoretical density (T.D.) in a 2.45 GHz multimode cavity. M. Janney and H Kimrey found that the alumina sample could achieve a density up to 98% T.D. at 1100xc2x0 C. when microwave sintered at 28 GHz, and they suggested that compared to conventional sintering processing, the sintering activation energy is much lower when microwave radiation is applied, which leads to higher densification rate at lower temperatures in a microwave field. One of the inventors of the present invention, J. Cheng, and his co-workers investigated the densification kinetics of alumina, and found that the diffusion coefficient during microwave sintering was three times higher than that in the conventional sintering at the same temperature. None of these efforts resulted in fully dense, and therefore optically transparent alumina articles.
In U.S. Pat. No. 5,451,553, Scott et al. describe a solid state process for the conversion of polycrystalline alumina to sapphire material. In the described process, a polycrystalline material containing less than 100 ppm by weight of magnesia was reheated to temperatures above 1100xc2x0 C., but below the melting point of alumina, in a high purity hydrogen atmosphere for 300 hours, or at 1880xc2x0 C. for 3-9 hours. While effective, the described process is too time consuming and expensive for the large scale production of sintered alumina with adequate transparency.
The first attempt to prepare transparent ceramic samples by microwave sintering processing was conducted by Y. Fang et al. in 1995-1996. Using specially synthesized precursor powder, transparent hydroxyapatite and translucent mullite samples were made using a microwave sintering technique.
In Japanese Laid-Open Patent Application No. 7-187760, laid open Jul. 25, 1995, a method for manufacturing artificial sintered gemstone is described. A synthetic-gemstone starting material powder, obtained by adding chromium oxide, titanium oxide, and/or other oxides to an alumina powder and a magnesia powder is molded, and the resulted molding is then sintered by being heated at 1300 to 1800xc2x0 C. with microwaves at 2.45 to 200 GHz in a reduced atmospheric pressure (vacuum) of 100 to 0.01 Pa to produce a synthetic gemstone. While this reference provides no details of the apparatus, certain characteristics can be discerned from the method described. At the reduced pressure of the vacuum, the specified frequency range is called for in order to attain adequate heating, probably by creating a plasma. Further, the reference provides no pre-heating of the molded material, and neither describes nor suggests the use of a hydrogen atmosphere.
Thus, there remains a need for an efficient, cost effective method and a structure for microwave sintering of polycrystalline alumina ceramics to a transparent body. The present invention is directed to this need.
The present invention provides a method and an apparatus for the formation of transparent ceramic bodies from polycrystalline alumina (Al2O3). The apparatus comprises an enclosed, insulated chamber to retain a workpiece for the application of microwave energy. The chamber comprises a single or multimode microwave cavity into which is mounted a quartz tube. An insulation material, transparent to microwave energy, is positioned within the quartz tube. A port for the introduction of hydrogen penetrates the cavity so that the microwave sintering of the workpiece is performed in an ultra-pure hydrogen atmosphere. The workpiece is preferably mounted on a refractory tube such as alumina for the microwave sintering process.
The apparatus just described is used to carry out the microwave sintering method of the invention. A starting Al2O3 powder with magnesia of 0.05% by weight is used to form a workpiece of the desired size and shape, such as for example by molding. The sample or workpiece is preferably formed by uniaxial press at 300 MPa pressure and calcined at 1100xc2x0 C. for two hours in a conventional furnace for debindering. The workpiece is then placed inside the microwave chamber previously described and sintered, for example, at 0.915 to 2.45 GHz in a single mode cavity or a multi-mode cavity at power levels of 1.5 kW to 6 kW. Ultrahigh purity hydrogen can be applied as a sintering atmosphere for sintering at ambient pressure. Typically, the heating rate is 150xc2x0 C. per minute in the single mode cavity and 100xc2x0 C. per minute in the multi-mode cavity. High density and translucency are obtained by microwave sintering at 1700xc2x0 C. for only 10 minutes, but sintering up to 30 minutes provides a more highly transparent alumina product. This method may be used to produce an alumina ceramic of Al2O3 composition and a corundum crystal structure. Further, this method may be used to produce an alumina based ceramic which has the xcex2-Al2O3 or magneto-plumbite crystal structure or MgO.(1-3)Al2O3 and the structure of spinel.
In a further aspect of the present invention, the transparent alumina product obtained from the process just described may then be subjected to another microwave sintering step in order to develop a single crystal product. The sane apparatus previously described is used for this further processing step. This further sintering step produces a corundum (or sapphire) product.
These and other features of the invention will be apparent to those of skill in the art from a review of the following detailed description along with the accompanying drawings.